Catalyst for the synthesis of alkyl mercaptan and process for the production thereof

ABSTRACT

The invention relates to an oxidic catalyst containing cesium and tungsten for the synthesis of alkyl mercaptans from alkanols and hydrogen sulfide, and to a process for the production of this catalyst, wherein the molar ratio of cesium to tungsten is &lt;2:1.

FIELD OF THE INVENTION

The present invention relates to an oxidic catalyst containing cesiumand tungsten for the synthesis of alkyl mercaptans from alkanols andhydrogen sulfide, and to a process for the production of this catalyst.

BACKGROUND OF THE INVENTION

Methyl mercaptan in particular is an industrially importantintermediate., e.g. for the synthesis of methionine as well as for thesynthesis of dimethyl sulfoxide and dimethylsulfone. Today, it isproduced predominantly from methanol and hydrogen sulfide by reaction ona catalyst of aluminum oxide. The synthesis of the methyl mercaptangenerally takes place in the gas phase at temperatures of between 300and 500° C. and under pressures of between 1 and 25 bar.

Apart from the methyl mercaptan formed, the reaction mixture containsthe unreacted starting substances and by-products, such as e.g. dimethylsulfide and dimethyl ether, as well as the gases that are inert in thecontext of the reaction, such as e.g. methane, carbon monoxide, hydrogenand nitrogen. The methyl mercaptan formed is separated from thisreaction mixture.

For the economic efficiency of the process, the highest possibleselectivity is demanded in the catalytic reaction of methanol andhydrogen sulfide to form methyl mercaptan, in order to keep the costs aslow as possible for the separation of the methyl mercaptan formed fromthe reaction mixture. A major cost factor here is represented inparticular by the energy input for cooling the reaction gas mixture tocondense the methyl mercaptan.

To increase activity and selectivity, potassium tungstate or cesiumtungstate is conventionally added to aluminum oxide as the support. Thetungstate is generally used in quantities of up to 25 wt. %, based onthe total weight of the catalyst. An improvement in activity andselectivity is also obtained by increasing the molar ratio of hydrogensulfide to methanol. Molar ratios of between 1 and 10 are conventionallyused.

However, a high molar ratio also means a high excess of the hydrogensulfide in the reaction mixture and thus the need to circulate largequantities of gas. To reduce the energy input required for this, theratio of hydrogen sulfide to methanol should therefore deviate from 1only slightly.

U.S. Pat. No. 2,820,062 relates to a process for the production oforganic thiols, in which a catalyst of active aluminum oxide is used, towhich potassium tungstate has been added in a quantity of 1.5 to 15 wt.%, based on the weight of the catalyst. With this catalyst, goodactivities and selectivities are achieved at reaction temperatures of400° C. and molar ratios of 2. This US Patent Specification mentionsvarious ways of introducing the potassium tungstate into the aluminumoxide. Thus, impregnation processes, co-precipitations and pure mixingsare mentioned as being applicable. Little importance is attached to theactual production of the catalyst for the economic efficiency of theprocess for methyl mercaptan synthesis.

In EP 0 832 687 B1, the advantages of using cesium tungstate (Cs₂WO₄)instead of potassium tungstate (K₂WO₄) as promoter are described. Thus,by using cesium tungstate, increased activity can be achieved with goodselectivity at the same time.

By increasing the cesium tungstate concentration to up to 40 wt. %, theselectivity towards methyl mercaptan can be increased to up to 92%without the activity being disproportionately impaired.

According to the general opinion, the best selectivity is achieved withcatalysts in which the alkali/tungsten ratio equals 2:1 (A. V. Mashkinaet al., React. Kinet. Catal. Lett., vol. 36, No. 1, 159-164 (1988)).

SUMMARY OF THE INVENTION

The object of the present invention is to provide a catalyst and aprocess for the production thereof, which is distinguished by improvedactivity and selectivity with low molar ratios of hydrogen sulfide tomethanol compared with the known catalysts, and thus leads to bettereconomic efficiency of the process.

This object is achieved by the provision of a catalyst containing acatalytically active oxidic composition of cesium and tungsten with amolar ratio of cesium to tungsten of <2:1, particularly <2:1 to 0.8:1,preferably 1.9:1 to 1:1, especially 1.6:1 to 1:1.

DETAILED DESCRIPTION OF THE INVENTION

The oxidic composition can be described by the formula Cs_(x)WO_(y), inwhich x signifies <2 to 0.8 and y signifies 3.4 to <4.

The catalyst contains this composition in a quantity of 15 to 45 wt. %,particularly 20 to 36 wt. %, preferably >25 to 36 wt. %. In the case ofa core-shell catalyst, these proportions relate to the composition ofthe shell.

The composition of cesium and tungsten can be impregnated directly on toa support body to produce the catalyst in the form of a supportedcatalyst. In the production of catalysts in the form of extrudates orpellets, the powdered support is impregnated or mixed with the oxidiccomposition and the intermediate obtained is then shaped. If acore-shell catalyst is being produced, the powdered support isimpregnated with the catalytically active composition and the resultingmixture is then applied on to an inert support core in the form of ashell.

The support materials can also contribute to the catalytic activity inindividual cases.

The Cs/W ratio is preferably <1.9:1 to 1:1. Thus, the catalystsaccording to the invention for the reaction of alkanols with hydrogensulfide to form alkyl mercaptans contain a hyperstoichiometricproportion of tungsten compared with the catalyst from the prior artimpregnated with cesium tungstate (Cs₂WO₄).

It is clear that this higher proportion on the preferably used aluminumoxide imparts improved activity and, at the same time, improvedselectivity to the catalyst compared with the stoichiometric alkalineearth or alkali tungstate exclusively used in the prior art. Whereasincreasing the concentration of cesium tungstate (Cs₂WO₄) causes anincrease only in the selectivity of the catalyst with, at the same time,lower activity, when exclusively the tungsten content is increased andthe cesium content is unchanged, a further increase in selectivity isunexpectedly demonstrated with, at the same time, increased activity.

According to the invention, excellent selectivity can be achieved withvery high loadings with the promoter, without the activity of thecatalyst decreasing, as known from the prior art. Furthermore, it hasbeen found that the activity and selectivity of the catalyst can beadjusted in a targeted manner by means of the alkali-tungsten ratio. Thecatalyst is used in the form of a supported catalyst, in which thesurface is impregnated with the catalytically active substance, or acore-shell catalyst, in which a core is surrounded by a mixture ofcatalytically active substance and support material. Furthermore,extrudates or pellets in which the catalytically active substance ismixed with the powdered support material before being shaped, or thelatter is impregnated therewith, can be used (uniform catalyst).

The known oxidic, inorganic compounds are used as support materials forthis catalyst, such as e.g. SiO₂, TiO₂, ZrO₂ and preferably so-calledactive aluminum oxide. This material exhibits high specific surfaces ofbetween about and 400 m²/g and consists mainly of oxides of thetransition series of the crystallographic phases of aluminum oxide (cf.e.g. Ullmann's Enzyclopedia of Industrial Chemistry, 1985, vol. A1,pages 561-562). These transition oxides include γ-, δ-, η-, κ-, χ- andθ-aluminum oxide. All these crystallographic phases are converted to thethermally stable α-aluminum oxide when the aluminum oxide is heated totemperatures of more than 1100° C. Active aluminum oxide is commerciallyavailable for catalytic applications in various grades and physicalforms.

Particularly suitable for the production of supported catalysts areformed pieces of granulated or extruded aluminum oxide with particlediameters of 1 to 5 mm, a specific surface of 180 to 400 m²/g, a totalpore volume of between 0.3 and 1.2 ml/g and a bulk density of 300 to 900g/l. For the purposes of the invention, aluminum oxide with more than200 m²/g specific surface is preferably used, since the catalyticactivity of the finished catalyst increases slightly with increasingsurface area of the aluminum oxide. This material is used in powderedform, preferably for the production of the core-shell catalysts,extrudates or pellets.

The aqueous impregnating solution for applying the promoter can besimply produced from water-soluble Cs and tungsten compounds,particularly tungstic acid (H₂WO₄) and cesium hydroxide (Cs (OH).H₂O).For this purpose, for example tungstic acid is suspended in water anddissolved with the addition of a base and heating. Cesium hydroxide oranother cesium salt is also dissolved in water and combined with thesolution of the tungstic acid (promoter solution). Cesium salts, theanions of which can be driven off completely by heat treatment, such ase.g. nitrates, formates, oxalates, acetates or carbonates, arepreferably used. Inorganic and also organic bases are suitable forstabilizing this solution with a pH of 8 to 14.

Those bases that can be driven off completely by a final heat treatmentof the catalyst obtained after impregnation are preferably used. Thesebases include preferably ammonium hydroxide and organic bases,particularly amines. Compared with the prior art, the molar ratio of Csand W when preparing the aqueous impregnating solution is selected suchthat, in contrast to cesium tungstate (Cs₂WO₄) with a Cs/W ratio of 2 to1, a higher proportion of tungsten, i.e. a Cs to W ratio of less than 2to 1, particularly <1.9:1 to 0.8:1, is present. This leads to markedlyincreased activity and selectivity of the catalysts according to theinvention compared with the known catalysts, particularly with lowratios of hydrogen sulfide and methanol in the reaction gas.

For the application of the promoter solution, various impregnatingtechniques can be used, such as dip impregnation, spray impregnation,vacuum impregnation and pore volume impregnation, it being possible forthe impregnation to take place more than once. In the case of formedpieces, the selected impregnating method must enable the desired loadingquantity of the promoter to be applied with good uniformity over theentire cross section.

The promoter solution is preferably applied on to the formed pieces inone or two steps by spray or vacuum impregnation. In spray impregnation,the aqueous impregnating solution is sprayed on to the support bodies.In vacuum impregnation, reduced pressure is created using a vacuum pumpin a vessel filled with the formed pieces. By opening a hose connectionto the aqueous impregnating solution, the solution is sucked into thevessel until the entire charge of formed pieces is covered with thesolution. After an impregnating period of 0.2 to 2 hours, the solutionnot taken up by the material is discharged or poured off.

By pre-drying for a period of 1 to 10 hours at room temperature, theinitial concentration gradient over the cross section of the formedpieces can be largely equalized. Thus, the uniformity of theimpregnation over the cross section of the catalyst particles isimproved. The catalyst precursors thus obtained are preferably dried fora period of 1 to 10 hours at 100 to 200, preferably 100 to 140° C., toremove the residual moisture. A calcination then takes place for aperiod of 1 to 20, preferably 1 to 5 hours at 300 to 600, preferably 420to 480° C. As a result, the promoter is fixed on the aluminum oxide andthe base from the impregnating solution is destroyed and driven off. Agas stream may optionally flow through the charge of support bodies forthe catalyst precursors during the pre-drying, drying and calcining,which improves the removal of the residual moisture and decompositiongases.

The impregnation of the formed pieces can also take place in multiplesteps, particularly in two steps.

In a preferred embodiment, the solution used in the first step thencontains one to two thirds of the total quantity of cesium and tungstencompounds provided.

In a multiple-step, but particularly a two-step procedure, the precursorobtained in the first step is optionally not calcined.

Otherwise, the same impregnation, drying and calcination program takesplace in the second step as described for the one-step process.

This multiple-step impregnation is sensible particularly when highloadings are desired and/or the limited solubility of the promotermixture does not allow the loading to be performed in one step.

It is also possible to spray the support bodies with the impregnatingsolution several times during the impregnating operation (step a fromclaim 11) and to remove portions of the residual moisture between eachof these treatment steps at a temperature of up to 120° C. before goingon to step b.

In the production of the core-shell catalyst, the powder to be appliedas the shell can be calcined before or after coating. For example, thistype of catalyst can be produced in accordance with EP-B-0 068 193. Thecalcination can also take place before and/or after shaping in theproduction of extrudates or pellets.

For the following examples, the different types of commercial aluminumoxide listed in Table 1 were used.

TABLE 1 Properties of the aluminum oxide used Aluminum oxide I Aluminumoxide II Manufacturer Rhodia Alcoa Type Spheralite 501A LD 350 Specific310 350 surface [m²/g] Bulk density 690-790 590-660 [kg/m³] Waterabsorption 0.50 0.58 [ml/g] Particle diameter 2-5 1.2-2.4 [mm] Loss onignition 4.5 5.6 850° C. [wt. %]

Examples Comparative Example 1

150 g aluminum oxide I was impregnated with 21.0 wt. % cesium tungstate(Cs_(2.0)WO₄) by means of vacuum impregnation. The details of theprocedure were as follows:

To prepare the impregnating solution, 55.7 g of tungstic acid weresuspended in 44.5 g water and dissolved by adding 111.4 g of 25% ammoniasolution and heating to 50° C. 74.6 g Cs(OH).H₂O were dissolved in 37.3g water and mixed with the first solution. The solution was then stirredfor 48 hours in a covered beaker. The solution was then topped up with25 g water to a volume of 234 ml.

The aluminum oxide was placed in a glass vessel, which had beenevacuated to 150 mbar. By opening a tap, the impregnating solution wassucked into the evacuated glass vessel until the entire charge of formedpieces was covered with the solution. After waiting 15 minutes andletting air into the glass vessel, the solution not taken up by thealuminum oxide flowed back into the beaker. During this operation, 79 mlof impregnating solution were taken up by the aluminum oxide.

The granules were dried for a period of 1 hour at room temperature in anair stream and then at 120° C. for 3 hours to remove the residualmoisture. The granules were then calcined for 3 hours at 455° C.

Comparative Example 2

Comparative example 1 was repeated with a 26.3% loading of the aluminumoxides with cesium tungstate (Cs_(2.0)WO₄).

Example 1

150 g aluminum oxide I was impregnated with 23.4 wt. % promoter(Cs_(1.6)WO_(y)) by vacuum impregnation.

For this purpose, 71.9 g tungstic acid were suspended in 44.5 g waterand dissolved by adding 111.4 g of 25% ammonia solution and heating to50° C. 76.9 g Cs(OH).H₂O were dissolved in 37.3 g water and mixed withthe first solution. The solution was then stirred in a covered beakerfor 48 hours. The solution was then topped up with 19 g water to avolume of 234 ml.

Impregnation, drying and calcination were carried out as in precedingexamples.

Example 2

150 g aluminum oxide I was impregnated with 25.8 wt. % promoter(Cs_(1.3)WO_(y)) by vacuum impregnation.

For this purpose, 88.9 g tungstic acid were dissolved in 44.5 g waterand 177.7 g of 25% ammonia solution. 79.3 g Cs(OH).H₂O were then addedand the solution was heated to 50° C. The solution was then stirred in acovered beaker for 67 hours. The solution was then topped up with 5 gwater to a volume of 234 ml.

Impregnation, drying and calcination were carried out as in precedingexamples.

Example 3

150 g aluminum oxide I was impregnated in a two-step impregnation with atotal of 30.5 wt. % promoter (Cs_(1.0)WO_(y)) by vacuum impregnation.The details of the procedure were as follows:

63.3 g tungstic acid were suspended in 50.7 g water and dissolved byadding 126.5 g of 25% ammonia solution and heating to 50° C. 42.4 gCs(OH).H₂O were dissolved in 21.2 g water and mixed with the firstsolution. The solution was then stirred in a covered beaker for 66hours. The solution was then topped up with 40 g water to a volume of234 ml. The aluminum oxide was placed in a glass vessel that wasevacuated to 150 mbar. By opening a tap, the impregnating solution wassucked in until the entire charge of formed pieces was covered with thesolution. After waiting for 15 minutes and letting air into the glassvessel, the solution not taken up by the aluminum oxide flowed back intothe beaker. During this operation, 76 ml of impregnating solution weretaken up by the aluminum oxide. The granules were then dried for 1 hourat room temperature and then at 120° C. for 3 hours, and calcined for 3hours at 455° C. As a result of this treatment, 18.7 wt. % promoter hadbeen deposited on the catalyst particles.

To perform the second impregnation, an impregnating solution identicalto that in the first step was prepared and applied on to the alreadyloaded catalyst from the first step in the same way by vacuumimpregnation. This was then followed by drying again for 1 hour at roomtemperature, followed by a 3-hour drying at 120° C. Finally, thecatalyst particles were calcined for 4 hours at 455° C. in air.

Example 4

Example 3 was repeated with a 31.4% loading of the aluminum oxide withcesium tungstate (Cs_(1.4)WO_(y)).

Example 5

Example 3 was repeated with a 34.5% loading of the aluminum oxide withcesium tungstate (Cs_(1.6)WO_(y)).

Example 6

Example 3 was repeated with a 33.2% loading of the aluminum oxide withcesium tungstate (Cs_(1.4)WO_(y)). Instead of the aluminum oxide I,however, aluminum oxide II was used.

Example 7

300 g aluminum oxide I was impregnated with a total of 35.3 wt. %promoter (Cs_(1.4)WO_(y)) by spray impregnation.

To prepare the impregnating solution, 95.1 g tungstic acid weresuspended in 76.0 g water and dissolved by adding 190.1 g of 25% ammoniasolution and heating to 50° C. 90.2 g CsOH.H₂O were dissolved in 45.1 gwater and mixed with the first solution. The solution was then stirredin a covered beaker for 48 hours. The granules were sprayed with theimpregnating solution while being rolled round in a coating pan. Since,owing to the limited water absorption, the granules could not take upthe entire liquid volume of the impregnating solution, the granules wereheated several times to 110° C. with a hot air gun between spraying withthe impregnation in order to remove portions of the residual moisture.

The granules were then stored in air for a period of 1 hour and thendried at 120° C. for 3 hours to remove the residual moisture. Thegranules were then calcined for 3 hours at 455° C.

Example 8

150 g aluminum oxide I was impregnated with 31.1 wt. % promoter(Cs_(1.4)WO_(y)) by vacuum impregnation.

For this purpose, 123.0 g tungstic acid were suspended in 230.0 g of 25%ammonia solution and 116.2 g CsOH.H₂O were added. Immediately afterheating to 50° C., the still hot impregnating solution was applied on tothe aluminum oxide by vacuum impregnation as described in ComparativeExample 1.

Drying and calcination were carried out as in previous examples.

Application Example

The catalysts were tested with respect to their performance data in thesynthesis of methyl mercaptan from hydrogen sulfide and methanol.

The synthesis was performed in a stainless steel tube with an 18 mminternal diameter and a length of 500 mm. The catalyst bed of 76 ml ineach case was fixed in the reaction tube on both sides by inert beds ofglass beads. The reaction tube was heated to the reaction temperature ofabout 320° C. using a double-walled jacket with a thermo oil.

The test conditions can be taken from the following list:

GHSV: 1300 h⁻¹ (based on standard conditions) LHSV: 0.84 h⁻¹ (based onliquid MeOH) Reaction temperature: 320° C. Weight ratio H₂S/MeOH: 1.9Pressure: 9 bar

The reaction mixture with the products methyl mercaptan, dimethylsulfide and dimethyl ether and with the unreacted starting substancesmethanol and hydrogen sulfide is analyzed by online gas chromatography.

The results of the measurements can be taken from Table 2. As theresults show, the increase in the cesium tungstate (Cs_(2.0)WO₄) loadingimproves the selectivity, but the activity and the yield deteriorate asa result (Comparative Examples 1 and 2).

If the proportion of tungsten in the catalyst is increased in relationto the proportion of cesium, a marked increase in the activity can beseen with, at the same time, improved selectivity. This leads to anincrease in yield of up to 13% compared with the prior art. Theselectivity can be raised by increasing the tungsten content and byincreasing the total loading to more than 30 wt. % up to about 97%, witha simultaneous rise in methanol conversion. In the industrial synthesisof methyl mercaptan, this also leads to considerable cost savings in theseparation of the reaction product from unreacted methanol andby-products.

TABLE 2 Test results Molar Methanol Aluminum ratio Loading conversionSelectivity Yield Catalyst oxide Cs:W [wt. %] [%] [%] [%] CE1 I   2:121.0 82.4 93.3 76.9 CE2 I   2:1 26.3 79.5 94.7 75.2 Ex. 1 I 1.6:1 23.485.0 94.9 80.7 Ex. 2 I 1.3:1 25.8 90.1 94.9 85.5 Ex. 3*⁾ I 1.0:1 30.597.6 92.0 89.8 Ex. 4*⁾ I 1.4:1 31.4 89.1 96.1 85.6 Ex. 5*⁾ I 1.6:1 34.585.1 97.0 82.5 Ex. 6*⁾ II 1.6:1 33.2 91.6 96.9 88.8 Ex. 7*⁾ I 1.4:1 35.585.9 95.9 82.4 Ex. 8 I 1.4:1 31.1 88.6 96.2 85.2 CE1: catalyst accordingto Comparative Example 1 *⁾multi-step impregnation

1-31. (canceled)
 32. A process for the production of a cesium-tungstencatalyst composition comprising the steps of: a) impregnating a supportbody or a support material with an aqueous solution of soluble cesiumand tungsten at a molar ratio of cesium to tungsten of less than 2:1, toform an impregnated support, b) pre-drying the impregnated support atroom temperature, c) optionally further drying the pre-dried impregnatedsupport at about 100 to about 200° C. to remove any residual moisture,and d) calcining the pre-dried or dried impregnated support for a periodof about 2 to about 10 hours at a temperature ranging from about 300 toabout 600° C.
 33. The process according to claim 32, further comprisingthe step of recovering the catalyst composition.
 34. The processaccording to claim 33, wherein the recovered catalyst compositioncontains the support material impregnated with the catalyst composition.35. The process according to claim 32, further comprising the step ofsuspending the impregnated support through the addition of one or moreauxiliary substances.
 36. The process according to claim 35, furthercomprising the step of applying the suspended impregnated support ontoan inert support core.
 37. The process according to claim 35, furthercomprising the step of extruding and compressing the suspendedimpregnated support.
 38. The process according to claim 32, wherein thesteps (a) to (c) and optionally step (d) are repeated at least once. 39.The process according to claim 38, wherein, when said process is carriedout more than once, the impregnating solution used in the first set ofsteps contains about one-third to about two-thirds of the total amountof cesium and tungsten.
 40. The process according to claim 38, furthercomprising, after the impregnation step, the step of spraying thesupport bodies or support material at least once with the aqueoussolution.
 41. The process according to claim 40, further comprising,after the spraying step, the step of removing at least part of theresidual moisture on the support bodies or support material by heatingthe support bodies or support material at a temperature of up to about120° C.
 42. The process according to claim 36, further comprising thestep of tempering the catalyst composition.
 43. A process for theproduction of alkyl mercaptans, comprising the step of reacting one ormore alkanols with hydrogen sulfide in the presence of the catalystcomposition according to claim
 19. 44. The process according to claim43, wherein the alkyl mercaptans are methyl mercaptans produced by thereaction of methyl alcohol and hydrogen sulfide.